In principle, the prior art discloses two different types of PEM fuel cell systems (PEM=polymer electrolyte membrane). In this context, a distinction is drawn between fuel cell systems having a gas generation device and fuel cell systems which are operated directly with hydrogen. In the case of fuel cell systems with a gas generation device, a hydrogen-containing gas is generated in the gas generation device in order to operate the fuel cells. For this purpose, a hydrocarbon-containing compound, for example alcohol, gasoline or diesel, together with water and if appropriate air is generally converted into a hydrogen-rich gas and carbon dioxide.
In both the fuel cell systems described above, the air fed to the cathode region of the PEM fuel cell, or another oxygen-containing medium, has to be suitably humidified in order to protect the PEM from drying out. In addition, in fuel cell systems having a gas generation device, the water balance of the fuel cell system must as far as possible be continuous, in order on the one hand to ensure that the PEM is moistened and on the other hand to provide sufficient water for operation of the gas generation device without the water having to be constantly topped up. The water, stored in particular in liquid form, is then used for hot-steam reforming or auto thermal reforming of the hydrocarbon-containing compound used to obtain hydrogen in the gas generation device.
In the PEM fuel cell itself, an anode space, to which the hydrogen or hydrogen-containing gas is fed, is separated from the cathode space, to which an oxygen-containing medium, in particular air, is fed, by the PEM, which is usually installed as part of a membrane electrode assembly (MEA). In the fuel cell, in structures which are currently customary, water is simultaneously transferred from the anode side to the cathode side of the PEM. Likewise, the product water is formed during the reaction of hydrogen and oxygen. This product water is usually discharged from the cathode space by an exhaust-gas stream.
U.S. Pat. Nos. 6,007,931 and 6,048,383 now describe corresponding processes in which the humid exhaust-gas stream from the cathode space is passed through a humidifier device which has a membrane permeable to water vapor. The water vapor contained in the exhaust-gas stream can in this way pass through the membrane and humidify a gas stream flowing in the humidifier device on the other side of the membrane. This gas stream may in particular be the air supplied in the cathode space.
In this method, a dew point which is substantially dependent on the temperature of the cathode exhaust gas, the transfer capacity of the membrane and the load point of the fuel cell system is established in the gas stream that is to be humidified and is flowing to the cathode space. However, at a relatively high membrane transfer capacity, such high dew points may be established in the gas stream that is to be humidified and is fed to the cathode space that reliable operation of the fuel cell is no longer ensured. Instead of useful humidification, the result in practice is “flogging” of the PEM or the electrodes/catalysts arranged in the region thereof and/or of the cathode space, and the electric power of the fuel cell therefore drops.